1. Field of the Invention
The present invention relates to an antenna suitable for use in a portable radio device and the like, and more particularly, to an antenna which resonates at a plurality of frequencies.
2. Description of the Related Art
A portable radio device represented by a portable telephone device or a cellular telephone device has an antenna mounted on the top of its housing, to radiate or capture radio waves through the antenna for transmission and reception operations. A portable telephone is provided with an antenna which is made up of an upper helical antenna and a lower whip antenna (or rod antenna). The antenna is arranged telescopically to and from a housing, such that a gain is increased when the antenna is extended, while the antenna protrudes less from the housing when it is retracted. Also, some recent portable telephones support a plurality of radio communication systems, and an antenna used in such a portable telephone must have characteristics of resonating in a plurality of frequency bands.
As an antenna which resonates in a plurality of frequency bands and can operate in these frequency bands, Japanese Patent Laid-open Application No. 10-22730 (JP, 10022730, A), for example, discloses a helical antenna which has coaxially arranged windings spirally wound at different winding pitches, as illustrated in FIG. 1A. In the helical antenna illustrated in FIG. 1A, first helical antenna element 91 and second helical antenna element 92, each of which is formed by helically wounding a conductive wire, are accommodated in cover 93. First helical antenna element 91 has a smaller winding pitch than second helical antenna element 92. These antenna elements 91, 92 are coaxially arranged, and connected respectively to mounting metal fixture 93 at one end thereof. FIG. 1B illustrates an equivalent circuit of this conventional helical antenna, where first helical antenna element 91 is connected directly to second helical antenna element 92, and both helical antenna elements 91, 92 are fed by feeding unit 90.
With a helical antenna, a winding pitch and the number of turns are varied for adjusting the characteristics of the antenna. For example, a wider winding pitch results in a wider bandwidth at a resonant frequency. In this event, if the winding pitch is varied without changing the overall length of the antenna, the number of turns is changed, leading to a change in an electric length of the antenna element, and an eventual change in the resonant frequency. Moreover, in the helical antenna illustrated in FIGS. 1A and 1B, two helical antenna elements 91, 92 are directly connected by an electric conductor, that is, they are connected through a DC (direct current) path, so that if one of the helical antenna elements is varied in structure (the winding pitch and/or number of turns) to change the antenna characteristics, this causes a change in the characteristics of the other helical antenna element, with the result that difficulties are encountered in adjusting both elements to respective optimal characteristics.
Also, in the conventional helical antenna, each helical antenna element 91, 92 is always connected to the feeding unit at one end thereof, so that it resonates with a quarter wavelength. Therefore, for resonating the helical antenna in different resonant modes, a matching circuit must be provided for each frequency. When this helical antenna is attached to a leading end of a rod antenna, this antenna can be applied only at frequencies which are in a particular relationship. As a specific example, where the wavelength is represented by λ, the helical antenna can be applied only to an antenna which resonates at λ/4 for frequency of 1.5 GHz and at 3λ/8 for frequency of 1.9 GHz. Particularly, when one frequency is substantially twice as high as the other frequency, the half wavelength resonance condition is difficult to realize at any frequency because the antenna element is powered from one end thereof.